Five to 10% of people taking ampicillin develop eruptions between the 5th and 14th day following initiation of therapy. Also, there is a 95% incidence of exanthematous eruptions in patients who are treated for infectious mononucleosis with ampicillin. The allergenicity of ampicillin appears to be enhanced by allopurinol or by hyperuricemia. Ampicillin is clearly the more allergenic of the two drugs when given alone.

Used in combination with sulbactam (as Unasyn).